Communications users, particularly telecommunications users, have required ever-increasing ranges of information transport. In the traditional telephone network, voice signals were transmitted and switched through the network in analog form. Because of economies in certain types of transmission media, voice signals were digitized for transmission purposes. Time-division multiplexing of digital voice signals was the most economical way to utilize the wire-based transmission plant of the telephone network.
With the advent of data processing and distributed data processing systems, a need arose for the transmission of data over communications links and through the telephone network. For purposes herein, "data communications" is broadly defined as any information transmitted through a digital communications network other than digitized voice signals. Currently, the most common type of data communications is alphanumerical data, i.e., text or numerical data. Future communications requirements include the ability to carry image and video communications in substantial proportions. Image communications is the transmission of a still picture or motionless object. Facsimile transmission, presently the most common form of image communications, is the transmission of the image of a block or page of information rather than transmission of the digital representations of the letters or characters which comprise the block or page. Video transmission adds motion to image transmission. It can range from transmission of full motion color television signals to freeze-frame video, which is a series of sequential still images. As image and video communications become more prevalent, the demand for bandwidth will increase dramatically. No doubt, there will be even greater communications demands in the future, both as to diversity of services and traffic capacities.
It is well settled that digital time-division multiplexed transmission is preferred for both voice and data communications for a number of reasons not the least of which are the substantial economies realizable from digital multiplexing. Digital multiplexing can occur between communications of the same type, such as interleaving a plurality of voice conversations onto a single pair of wires. Digital multiplexing can also occur between communications of different types, such as inserting data communications into detectable silence periods in voice communications. Such detectable silence periods may occur while one conversant is listening or in gaps between words or syllables of a speaker. Digital multiplexing is particularly suited to adapting to variable bandwidth demands which result from the inherently "bursty" nature of most voice and data communications. Thus, integration of voice and data is spurred by the substantial economies of digital multiplexing and the growing diversity of services.
A digital communications network or system is said to be "integrated" or to provide "integrated services" if the network or system has the capacity to transmit voice and data communications through common equipment and facilities. An attribute of integrated communications systems is the use of intelligent processors at various points in the network for control purposes. Control is "distributed" or "dispersed" if the overall network control emanates from multiple geographical points, each point using local information or information provided by distant points via the network itself. Thus, the intelligence in a distributed control network is dispersed throughout the geographical area being served. In particular, a switching decision which needs to be made by a local processor can be made with information immediately available to the local processor. In large communications systems, distributed control generally improves efficiency since the intelligence required to route local traffic is nearby. Distributed control also enhances survivability since a local portion of the system, being self-controlled, will remain operable in the event a distant control point should be out of service.
With the ever-increasing demand for transmission bandwidth, it is axiomatic that higher bit rates will be employed over communications links in the future. On the Bell System T1-carrier, of which millions of miles are already installed, a communications link carries 1.544 million bits per second. Links with substantially higher bit rates are feasible even with current technology. The provision of integrated services over high-speed communications links will require new methods, procedures, and protocols governing information transport through the network. In particular, additional bandwidth required by the system for routing and administration, i.e., the "overhead," should be minimized while permitting reasonable flexibility within the network to adapt to changing circumstances. Integrated switching apparatus should be capable of transmitting and routing information at T1 rates and higher, so that optimal channel utilization can be achieved.
In endeavors to provide integrated services in a single network or even within the same switch, various combinations of existing technologies have been made. One approach has been to combine techniques of packet and circuit switching. This approach has resulted in various hybrid techniques employing combinations of hardware, software, or both. As costs associated with fast processing has diminished, voice and data traffic have become increasingly suited to being transmitted in streams of discrete digital packages. This type of traffic does not ideally match the capabilities of either packet or circuit switching. When a stream of discrete packages is transmitted through a circuit-switched network, the transmission capacity between packages generally is wasted. When such a stream is transmitted through a packet-switched network, the transmission capacity between packets is utilized by the system but a percentage of the transmission capacity within each packet is dedicated to administration. This percentage of overhead is relatively high, particularly where speech is concerned because in speech transmission packet size is usually reduced in order to minimize delay, etc. In a small packet, even the minimum number of bits dedicated to administration results in a high overhead percentage.
It would advance the art if a communications method were provided which is capable of handling both voice and data traffic in a highly efficient manner without the problems associated with either packet or circuit switching. Such a method should be flexible and dynamic so as to utilize virtually all of the available transmission capacity. On the other hand, the method should consume minimal transmission capacity for system administration.